The present invention relates to a printing apparatus for printing an image by using a printhead having a plurality of printing elements, a control method of the apparatus, and a computer-readable memory.
Note that the present invention is applicable not only to a general printing apparatus but also to a copying machine, a facsimile apparatus having a communication system, a word processor having a printing unit, and an industrial printing apparatus combined with various processors.
Printing apparatuses such as a printer, a copying machine, and a facsimile apparatus are so constructed as to print an image composed of dot patterns on a printing medium such as a paper sheet or thin plastic plate on the basis of image information. Printing apparatuses like this can be classified, based on the type of printing system, into an inkjet system, a wire dot system, a thermal system, and a laser beam system. The inkjet system (inkjet printing apparatus) prints an image by discharging ink (printing solution) droplets from discharge orifices in a printhead and depositing the ink droplets on a printing medium.
Recently, a large number of printing apparatuses have been used, wherein high-speed printing, high resolution, high image quality and low noise are required. The above inkjet printing apparatus is an example of a printing apparatus meeting these requirements. This inkjet printing apparatus prints an image by discharging ink from a printhead, so noncontact printing is possible. Hence, the inkjet printing apparatus can form stable printed images on a wide variety of printing media.
Of such inkjet printing apparatuses, an apparatus using a method of printing an image by forming ink droplets by using thermal energy is simple in structure and hence has the advantage that the density of nozzles for discharging ink can be readily increased.
However, an inkjet printing apparatus requires stable discharge of ink because the apparatus prints an image by discharging ink from a printhead. That is, the printhead of an inkjet printing apparatus must have stable performance with respect to durability, environment, printhead temperature, number of simultaneously discharged ink droplets, and the like.
xe2x80x9cStable dischargexe2x80x9d means a stable discharge amount, discharge speed, and discharge state (ink droplet landing position).
For this stabilization, control has been performed such that a driving pulse to be applied to a printhead is changed in accordance with the temperature of a printing apparatus main body or of the printhead.
In conventional apparatuses, the number of discharge energy generating elements to be simultaneously driven changes in accordance with an image to be printed, so an electric current flowing from the power supply of the main body fluctuates. This changes the voltage drop resulting from the resistance of a wire connecting the main body and the printhead. Therefore, when a predetermined voltage is applied to the printhead, the voltage applied to the discharge energy generating elements in the printhead fluctuates for each image to be printed.
For example, in a general inkjet printing apparatus, the wiring resistance between the main body and the printhead is about 0.2xcexa9, and the head contact resistance is about 0.1xcexa9, so the total resistance is about 0.3xcexa9. Assume that a driving current of 100 to 200 mA flows through each discharge energy generating element and twenty-four elements are simultaneously driven, the total current is 2.4 to 4.8 A, and the voltage drop by the wiring is as high as 0.3xcexa9xc3x97(2.4 to 4.8 A)=0.72 to 1.44 V. This is the voltage fluctuation applied to the discharge energy generating elements.
This voltage fluctuation applied to the discharge energy generating elements results in fluctuations in discharge energy, i.e., fluctuations in ink discharge amount and discharge speed. Consequently, printing density unevenness or ink droplet landing position deviation takes place, or no ink droplet is discharged. This significantly degrades the printing quality.
Also, the number of simultaneous ink discharging nozzles changes the voltage to be applied to a heater of each nozzle of the printhead. A driving voltage and a driving pulse are so determined that ink droplets are stably discharged even when the number of simultaneous ink discharging nozzles is a maximum, i.e., even when the driving voltage is a maximum. When the number of simultaneous ink discharging nozzles is small, therefore, an excess driving voltage or an excess pulse width of a driving pulse is applied to the heaters, and this degrades the durability.
To solve these problems, diverse methods have been conventionally proposed.
For example, Japanese Patent Laid-Open No. 58-5280 has proposed a thermal dot printing apparatus which changes the driving pulse width and the driving interval in accordance with the number of dots to be simultaneously driven.
Also, Japanese Patent Laid-Open No. 5-96771 has proposed a thermal transfer printing apparatus which changes the driving time by detecting the number of resistors to be powered in order to correct the voltage drop in a common wiring portion.
Japanese Patent Laid-Open No. 5-116342 has proposed an inkjet printing apparatus in which a detecting unit using an MPU and a RAM detects the number of dots to be simultaneously discharged and the driving voltage is controlled by using the detection result.
Japanese Patent Laid-Open No. 9-11463 has proposed an inkjet printing apparatus in which an image signal from a host apparatus or the like is temporarily stored in a buffer and converted into a bit signal for each heat-generating resistor in an inkjet printhead by an image processing circuit, and the driving pulse conditions are determined by using a lookup table on the basis of the number of dots to be discharged, the positions of nozzles, and temperature information obtained from a thermistor added to the inkjet printhead.
Furthermore, Japanese Patent Laid-Open No. 9-11504 has proposed an inkjet printing apparatus which counts the number of nozzles to be simultaneously driven in a printhead, stores driving parameters in a RAM on the basis of this counted value, and uses these stored driving parameters.
The foregoing are the conventional examples of driving pulse width control for improving the printing stability and the conventional examples of driving pulse width control based on the number of simultaneous ink discharging nozzles. As described above, driving pulse width control is performed for various purposes. When the driving pulse width is changed in accordance with the temperature, the voltage drop by simultaneous discharge also changes. This complicates the process of appropriate control. Also, while the driving frequency of a printhead increases from ten-odd KHz to several tens of KHz with recent improvements in printing speed and printing quality, the amount of printing data is increasing. This makes the conventional method of previously counting printing data difficult to perform.
The present invention has been made in consideration of the above problems, and has as its object to provide a printing apparatus capable of stable printing, a control method of the apparatus, and a computer-readable memory.
A printing apparatus according to the present invention, for achieving the above object, has the following arrangement.
A printing apparatus for performing printing by using a printhead having a plurality of printing elements comprises discriminating means for discriminating the number of simultaneously driven printing elements of the plurality of printing elements when printing data is printed, and control means for controlling a driving pulse to be applied to printing elements used in the printing of the printing data, on the basis of a fundamental pulse width. The fundamental pulse width is variable and is determined on the basis of driving conditions of the printhead and the number of simultaneously driven printing elements discriminated by the discriminating means.
Preferably, the driving conditions include a wiring resistance, heater resistance, driving TrON resistance, and environmental temperature of the printhead.
Preferably, the control means comprises storage means for storing a first management table for managing the correspondence of the driving conditions with the fundamental pulse width, and a second management table for managing the correspondence of the fundamental pulse width with a change amount of the fundamental pulse width based on the number of simultaneously driven printing elements; first determining means for determining a fundamental pulse width corresponding to the driving conditions by referring to the first management table; and second determining means for determining a change amount of the fundamental pulse width, which corresponds to the number of simultaneously driven printing elements, by referring to the second management table, and changes the fundamental pulse width determined by the first determining means by the change amount determined by the second determining means to generate a driving pulse to be applied to printing elements used in the printing of the printing data.
Preferably, the control means defines the fundamental pulse width by one of leading and trailing edges of a pulse signal on the basis of the driving conditions, and controls a driving pulse width of a driving pulse to be applied to printing elements by the other, on the basis of the number of simultaneously driven printing elements.
Preferably, the control means comprises storage means for storing a third management table for managing the correspondence of rise time and fall time of the heat pulse, the driving conditions, and the fundamental pulse width, and controls a pulse width of the driving pulse corresponding to the number of simultaneously driven printing elements and the driving conditions by referring to the third management table.
Preferably, the printing apparatus comprises a plurality of printheads, and if power lines for supplying power to the printheads are independent of each other, the control means executes the control for each power line.
Preferably, the control means makes a change amount for the driving pulse, which the control means generates by changing a pulse width of the fundamental pulse when the number of simultaneously driven printing elements is equal to or larger than a predetermined value, smaller than a change amount for the driving pulse, which the control means generates by changing a pulse width of the fundamental pulse when the number of simultaneously driven printing elements is less than the predetermined value.
Preferably, the control means makes a change amount for the driving pulse, which the control means generates by changing a pulse width of the fundamental pulse when the number of simultaneously driven printing elements is equal to or smaller than a predetermined value, larger than a change amount for the driving pulse, which the control means generates by changing a pulse width of the fundamental pulse when the number of simultaneously driven printing elements is less than the predetermined value.
Preferably, if the number of simultaneously driven printing elements for use in predischarge of the printhead is limited, the control means makes a pulse width of a driving pulse applied to printing elements used in the predischarge larger than a pulse width of a driving pulse applied to printing elements for use in printing, which uses an equal or larger number of printing elements than the number of simultaneously driven printing elements used in predischarge.
Preferably, when predischarge of the printhead is to be performed, the control means applies a driving pulse having a predetermined width to printing elements used in the predischarge.
Preferably, the fundamental pulse width is a fundamental pulse width selected and determined from a plurality of fundamental pulse widths.
Preferably, the driving conditions are conditions including printhead characteristics.
Preferably, the second management table holds as an index value a change in fundamental pulse width which is based on the number of simultaneously driven printing elements.
Preferably, a fourth management table representing a relationship between the change in fundamental pulse width and the index value, the fourth management table being prepared for each printing mode.
Preferably, the printing mode is a mode for performing printing complementarily in accordance with a printing pass count.
A method of controlling a printing apparatus according to the present invention for achieving the above object has the following steps.
A method of controlling a printing apparatus for performing printing by using a printhead having a plurality of printing elements comprises the discrimination step of discriminating the number of simultaneously driven printing elements of the plurality of printing elements when printing data is printed, and the control step of controlling a driving pulse to be applied to printing elements used in the printing of the printing data, on the basis of the number of simultaneously driven printing elements discriminated in the discrimination step and a fundamental pulse width determined on the basis of driving conditions of the printhead.
A computer-readable memory according to the present invention for achieving the above object has the following program codes.
A computer-readable memory storing program codes for control of a printing apparatus for performing printing by using a printhead having a plurality of printing elements comprises a program code of the discrimination step of discriminating the number of simultaneously driven printing elements of the plurality of printing elements when printing data is printed, and a program code of the control step of controlling a driving pulse to be applied to the printing elements used in the printing of the printing data, on the basis of the number of simultaneously driven printing elements discriminated in the discrimination step and a fundamental pulse width determined on the basis of driving conditions of the printhead.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.